IANAL

Since law is, like medicine (especially brain surgery), a field where bad advice can lead to litigation, this is a "cover your ass" maneuver to avoid being sued by someone who takes the advice and gets in big trouble. In other words, "I am not a lawyer, so I may not have any idea what I am talking about. I think this is right, or else I wouldn't be posting it, but I could have made a mistake so don't trust me too much before talking to a lawyer."

Of course, lawyers can be wrong just as often as people in other professions; one wonders what sort of disclaimer they must use in their USENET posts.